This invention relates to copolyesters of p-hydroxybenzoic acid, hydroquinone and a C.sub.8 -C.sub.12 aliphatic dicarboxylic acid and more particularly to copolyesters of p-hydroxybenzoic acid, hydroquinone and azelaic acid.
The use of objects molded from synthetic polymers has expanded rapidly in the last several decades. In particular, polyesters have widely gained acceptance for general molding applications. Although most polyesters have mechanical properties suitable for general molding applications, most polyesters are not suitable for high strength service because the mechanical properties are not sufficiently high. One group of polyesters that is suitable for high strength service without the need of a reinforcing agent is a new class of polymers exhibiting a general overall balance of mechanical properties substantially enhanced over previous polymers. These polymers have been described by various terms, including "liquid crystalline", "liquid crystal", and "anisotropic". Briefly, the polymers of this new class are thought to involve a parallel odering of the molecular chains. The state wherein the molecules are so ordered is often referred to either as the liquid crystal state or the nematic phase of the liquid crystal state. These polymers are prepared from monomers which are generally long, flat and fairly rigid along the long axis of the molecule and have chain extending linkages that are either coaxial or parallel. For example, liquid crystal copolyesters have been prepared from the following fairly rigid molecular species: p-hydroxybenzoic acid, hydroquinone and terephthalic or isophthalic acid. Such copolyesters are generally high melting and intractable.
I have now found that tractable copolyesters of enhanced liquid crystallinity can be prepared from p-hydroxybenzoic acid, hydroquinone and a fairly flexible linear aliphatic dicarboxylic acid containing 8 to 12 carbon atoms. Optionally, some of the aliphatic dicarboxylic acid can be replaced with an aromatic dicarboxylic acid selected from the group consisting of isophthalic acid, terephthalic acid, 1,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid and 2,7-naphthalene dicarboxylic acid. Thus the copolyesters of the present invention are capable of forming a thermotropic melt phase at a temperature below about 340.degree. C., have an inherent viscosity of at least 0.3, and consist essentially of the following divalent radicals: ##STR1## wherein n is in the range of 6 to 10; wherein R is an aromatic radical selected from the group consisting of 1,3 phenylene, 1,4-phenylene, 1,5 naphthalene, 2,6-naphthalene and 2,7-naphthalene and wherein the range of radical A is from 10 to 70 mole percent of the copolyester and the range of radical C is from 5 to 100 mole percent of the total of radicals C and D.
The copolyesters of the present invention are prepared by an acidolysis procedure in which esters of monocarboxylic acids such as acetic acid are prepared by esterification of the phenolic hydroxy groups of hydroquinone and p-hydroxybenzoic acid and the esters are reacted with the linear aliphatic dicarboxylic acid or with a mixture of the aliphatic and aromatic dicarboxylic acids set forth hereinabove at a temperature in the range of about 250.degree. to about 340.degree. C. The esters can be prepared in situ by reaction of the phenols with acetic anhydride.
The linear aliphatic dicarboxylic acids are selected from the group consisting of suberic acid, azelaic acid, sebacic acid, undecanedioic acid and dodecandioic acid. Azelaic acid is the preferred acid.
The amount of aromatic dicarboxylic acid which can be introduced into the copolyester is determined by the requirement that the copolyester be capable of forming a thermotropic melt phase at a temperature below about 340.degree. C. Therefore with the more linear terephthalic acid molecule, the amount that can be introduced is generally more limited than is the case with isophthalic acid.
In general the copolyester should have an inherent viscosity of at least about 0.3 and more preferably at least about 0.5 in order to provide adequate mechanical properties. The inherent viscosity can, if desired, be increased to values of 0.7 or even higher using conventional techniques such as melt or solid state polymerization. The inherent viscosity is determined at 30.degree. C. with a solution of 0.5 gram of copolyester per 100 ml of a solvent composed of 60 weight percent phenol and 40 weight percent tetrachloroethane.
The enhanced liquid crystallinity of the copolyesters can be demonstrated from the optical anisotropy of the melt above the flow temperature and from the melt viscosity behavior versus rate of shear. The degree of ordering in the melt causes the melt viscosity to be extremely shear sensitive and the melt viscosity can reach extremely high values at low rates of shear depending on the degree of ordering of the polymer molecules in the melt. The shear sensitivity of melt viscosity of the copolyesters of the present invention occurs over a lower range of shear rates than that of wholly aromatic polyesters and shows that a greater degree of ordering takes place.
Advantageously the ratios of monomers in the copolyesters of the present invention can be selected to provide copolyesters which are tractable at a low temperature, the flow temperature at which they form a thermotropic melt phase being substantially below 340.degree. C. and to have a high long-term use temperature associated with a high glass transition temperature. Preferably the glass transition temperature is at least about 110.degree. C. Among the preferred compositions, are copolyesters containing from about 20 to about 45 mole percent of p-hydroxybenzoic acid units and more preferably compositions containing from about 20 to about 45 mole percent p-hydroxybenzoic acid units, wherein from about 5 to about 25 mole percent of the dicarboxylic acid units are provided by the linear aliphatic dicarboxylic acid and the remaining dicarboxylic acid units are isophthalic acid units.
The copolyesters of the present invention can contain nucleating agents, fillers, pigments, glass fibers, mineral reinforcing agents, antioxidants, stabilizers, plasticizers, lubricants, fire-retardants and other additives.
The copolyesters are useful for preparing molded objects, films, fibers and the like. For example, they may be injection molded by conventional means and they may be formed into fibers by melt spinning and subsequently drawn, and further processed according to conventional techniques.